The present invention relates to a system for controlling a motor and more particularly to improvements in a system for controlling the speed of an elevator which is driven by an AC motor.
An apparatus in which a high speed elevator is operated by using an induction motor to drive a cage and controlling the applied voltage to the motor to provide smooth operation characteristics is disclosed, for example, in U.S. Pat. No. 3,866,097. The outline of the apparatus will be described with reference to FIG. 1.
In the drawing, reference symbols R-S-T represents a three-phase AC power source, numeral 1 a three-phase induction motor having primary windings 1A to 1C connected to each phase of the AC power source R-S-T, numerals 2A to 2C thyristors, numerals 3A to 3C diodes which are connected in parallel with the thyristors 2A to 2C, respectively, and are connected between the AC power source phases R-S-T and the primary windings 1A to 1C of the motor 1, numeral 3 a center tapped transformer having a primary winding connected to the AC power source R-S-T and the secondary windings connected to an opposite end of the primary winding 1B of the motor 1 via thyristors 4A and 4B and having a center tap connected to the primary winding 1C, numeral 5 a speed detector or tacho generator which is coupled with a rotor shaft of the motor 1 for generating a speed signal 5a which is proportional to the rotational speed of the rotor, numeral 6 a drive pulley which is driven by the rotor of the motor 1, numeral 7 a main rope which is wound on the pulleys 6, numeral 8 and 9 a cage and a counter weight connected to the opposite ends of the main rope 7, numeral 10 a speed command generating circuit, numeral 11 an operational amplifier which amplifies the difference between the speed command signal 10a and the speed signal 5a and operates a trigger control circuit 12 or 13 in accordance with the difference. The trigger control circuits 12 and 13 are adapted to trigger-control the thyristors 2A to 2C and 4A, 4B, respectively.
When the motor 1 is operated as is done when the cage 8 is decelerated during a descent mode of operation under a heavy load, the speed signal 5a becomes greater than the speed command value 10a. The operational amplifier 11 provides an output to the trigger control circuit 13 so that the thyristors 4A and 4B are trigger controlled. Accordingly, since the thyristors 4A and 4B provide a center tapped single phase full wave rectifying circuit, a direct current flows through the primary windings 1B, 1C of the motor 1 in a direction designated by an arrow X. The motor 1 provides a braking torque corresponding to that required by the load so that the cage 8 is smoothly decelerated.
On the other hand, demands for saving energy have recently increased. One of the provisions for realizing the saving of energy is to decrease the output of the motor by decreasing the speed of the cage 8 when a smaller number of passengers use the elevator. However in order to reduce the speed at a power running by means of a control as shown in FIG. 1, the voltage is lowered to increase the slip in the motor. In order to lower the speed at braking, only direct current should be increased. This makes the efficiency at a low speed operation very adverse. The input is reversely increased so that saving in energy is not accomplished even if the output of the motor 1 is decreased by lowering the speed.